In U.S. Pat. No. 4,500,870, granted on Feb. 19, 1985, which is assigned to the assignee of the present patent application, there is disclosed an optoelectronic sensor for the angular or rotational position of a shaft. The sensor works in association with a light transmitter and a light receiver which are mounted in fixed positions. An element is mounted on the shaft for rotation therewith and includes a light reflecting track, a portion of which provides a light reflecting surface that completes the light transmission path between the transmitter and receiver. The track is constructed and positioned on the element so that, as the element rotates, the portion of the track within the light transmission path assumes a predefined sequence of positions, each of which is at a different distance from the transmitter and/or the receiver. Each different angular position of the shaft therefore establishes a light transmission path of a different length and produces a different level of light intensity at the receiver. By detecting the level of light intensity at the receiver, it is therefore possible to establish the rotational position of the shaft.
The angular position sensor of U.S. Pat. No. 4,500,870 was disclosed in association with sensing the position of a shaft in a utility meter. In this application, it was necessary to resolve ten angular ranges (corresponding to the digits "0" through "9") as the shaft rotated a full 360 degrees. In such an application, the angular position sensor of the patent application exhibited satisfactory linearity and resolution and therefore provided excellent performance.
However, there are some applications in which angular position of a shaft must be detected precisely over very small angles or rotation. For example, torque sensors may employ a shaft of known torsional characteristics which is anchored at one end and has the torque to be determined applied at the free end. The relative angular or rotational displacement between the two ends of the shaft is then measured to determine the value of the applied torque. This application requires detection of very small angular displacements with a high degree of precision and, therefore, linearity.
A number of problems are encountered when it is attempted to apply the angular position sensor of the above-identified patent in a device requiring precision determination of small angular displacement. First of all, it is found that small angular displacements produce changes in detected light intensity which are very small in comparison to the absolute intensity of light and are therefore difficult to detect accurately. In addition, although the slight non-linearity which may be present in the sensor does not affect accuracy significantly when relatively large angular displacements were involved, it represents a source of substantial error, when it is desirable to detect small changes in angular position.
In copending U.S. patent application Ser. No. 647,031 filed Sept. 4, 1984, which is assigned to the assignee of the present application, there is disclosed my concept for an apparatus for sensing rotational or angular position of a shaft. The apparatus incorporates two light transmission paths, each having a separate photodetector providing an electrical signal in response to received light, and these signals are differentially processed to obtain a signal representative of rotational position. Devices are provided in each of the light transmission paths to vary the light transmitted in the path, and these devices are mechanically coupled to the shaft so that the variations in the respective light paths are opposite. The differentially processed photodetector signals are unaffected by absolute position of the shaft, but are responsive only to deviations from a reference position, and the differentially processed signal exhibits a high degree of linearity that was previously unobtainable.
However, the embodiments of the invention disclosed in that patent application tended to rely on various mechanical means to achieve the opposite light transmission in the two paths and were, generally, not amenable to precise measurement of the position of the shaft that is rotated through a large displacement. Furthermore, it would be desirable to avoid dependence on mechanical means as much as possible, in order to optimize reliability and minimize maintenance requirements.
Broadly, it is an object of the present invention to provide an apparatus for detecting very small angular displacements of a shaft with a high degree of precision, while avoiding the shortcomings of the prior art.
It is another object of the present invention to provide a high precision optoelectronic rotational position sensor which achieves a high degree of linearity and high precision with very small angular displacements, yet can maintain the same precision, even with large angular displacements.
It is also an object of the present invention to provide an optoelectronic torque sensor which achieves a high degree of linearity and high precision.
It is further an object of the present invention to provide an angular position sensor which is simple and relatively inexpensive in construction, yet accurate, reliable and convenient in use.
In an illustrative embodiment demonstrating objects and features of the present invention, an apparatus for determining the rotational position of a shaft comprises opposed first and second disks mounted for relative rotation about a predefined axis, with one of the disks being mounted to the shaft for rotation therewith and the other of the disks being maintained in a reference position. First and second rows of spaced apertures are provided on each disk, which lie in concentric paths of different radial distance from the predefined axis, the first rows being at the same radial distance from the axis and second rows being at the same radial distance from the axis. These apertures are positioned so that apertures of the first rows move into alignment while apertures of the second rows move out of alignment as the disks are rotated relative to each other. A first beam of light of predetermined intensity is oriented so as to intersect the paths of the first rows of apertures, and a second beam of light of predetermined intensity is oriented so as to intersect the paths of the second rows of apertures. First and second photosensors are aligned respectively with the first and second beams of light, and the photosensors are positioned so that the disks are in front of the respective photosensor for each light beam. Each photosensor provides an electric output signal relative to the intensity of light sensed by it and the two output signals are differentially processed to provide an orientation signal representative of the relative rotational position of the disks.